Boron is an essential plant nutrient and is also thought to be necessary for the health of mammals. However, at high concentrations, boron can be toxic to both plants and mammals, affecting plant growth and reproductive and nervous systems. Boron occurs naturally in seawater and in some well waters and springs, and is also present in waste waters from manufacturing of metals, microelectronics, and fertilizers, for example. Accordingly, effective methods for removing boron from aqueous solutions are required.
Methods for removing boron from geothermal wastewater using selective ion exchange resins of the N-glucamine type have been described (Kabay et al. Reactive & Functional Polymers; 60 (2004); 163-170). N-methyl-glucamine-type cellulose derivatives have also been described for removal of boron from wastewater (Inukai et al. Analytica Chimica Acta 511 (2004); 261-265). Peterson (U.S. Pat. No. 3,856,670) teaches the removal of boron from aqueous solutions using ion exchange phenolic resins containing aromatic ortho-hydroxy carboxylic groups cross-linked with aldehydes. Other insoluble resins and supports for boron absorption are also known (see U.S. Pat. No. 2,813,838, for example), including N-methyl glucamine modified inorganic supports and N-methyl-glucamine modified terpolymers of glycidyl methacrylate with methylmethacrylate and divinyl benzene in spherical bead form (Kaftan et al. Analytica Chimica Acta; 547 (2005); 31-41; Bicak et al. Reactive & Functional Polymers 47 (2001); 175-184). Such boron adsorption techniques are expensive due to the cost of the ion exchange resin and supporting processes. Further, such methods do not remove basic salinity, so if the boron-containing water has high salinity, additional processes, such as RO or distillation, are necessary to lower the salinity.
Removal of boron from seawater also presents an environmental concern, since seawater typically contains about 4 to 7 ppm boron, in addition to a variety of water-soluble salts. Treatment of seawater has become a significant issue since it requires reduction of both salinity and boron. Traditional methods for purifying (desalinating) seawater for drinking and irrigation purposes utilize reverse osmosis (RO) membranes, which are effective at significantly reducing the concentrations of all dissolved ions in the seawater. Although the reduction of the majority of dissolved ions by polyamide reverse osmosis membranes is about 98% to about 99%, the rejection rate of boron by these membranes is much lower, typically in the 70%-90% range, and may be even lower at high feed water temperatures (greater than about 25° C.).
The significantly lower rejection rate of boron by polyamide membranes may be explained by the very low dissociation rate of boric species at neutral pH. However, this boric species dissociation rate increases with pH and reaches 50% dissociation at a pH of 8.6 to 9.8, depending on the ionic strength of the solution and the temperature (W. Stumm, et al. Aquatic Chemistry, John Wiley & Sons (1981)). Consequently, an increased boron rejection rate is achievable at high pH, thus making possible appreciable reduction of boron concentration by reverse osmosis.
Magara et al. (Desalination 118:25-34 (1998)) and Prats et al. (Desalination 128: 269-273 (2000)) describe methods for reducing boron concentration using two-pass reverse osmosis systems. In these systems, the pH of the permeate from the first pass is increased before it is passed through the RO membrane in the second pass in order to improve the boron rejection. The term “permeate” is known in the art to refer to reverse osmosis product water. Because the RO permeate from these systems has low salinity and low concentration of scale-forming ions, even adjustment of the pH to high levels does not result in scale formation.
An example of a similar methodology applied to high salinity water is described by Tao et al. (U.S. Pat. No. 5,250,185), which involves the application of a high pH RO processing method to oilfield-produced water. In order to prevent scaling of the reverse osmosis system by carbonate salts, the feed water is softened prior to adjustment of the pH to a level greater than 9.5. Tao et al. teach that the high pH is necessary to obtain the desired increase in boron rejection. Additionally, Mukhopadhyay (U.S. Pat. No. 5,925,255) describes the treatment of brackish and low salinity water by reverse osmosis, in which the hardness of the RO feed water is removed by a weak acid cation exchange resin.
Finally, U.S. Pat. No. 7,442,309 of Wilf et al. describes a desalination treatment method for high salinity, boron-containing liquid which includes a method of reducing the boron concentration. The method involves increasing the pH of the non-softened, high salinity liquid to about 8 to 9.5 and passing the pH-increased, non-softened, high salinity liquid through at least one reverse osmosis device. The resulting permeate has a boron concentration of less than about 2 ppm.
It would be desirable to be able to significantly reduce the concentration of boron in both low and high salinity aqueous solutions using straightforward processes that would be attractive due to lower operating costs and superior effectiveness relative to known methods.